The purpose of this study is to examine certain epithelial tissues that have the property of actively transporting Cl- and which act as osmoregulatory sites at the systemic or specific organ level. The epithelia used in these studies will be the opercular skin of the killifish Fundulus Heteroclitus, the rectal gland of the dogfish, Squalus acanthias, and the cornea and pigment epithelium of the frog, Rana catesbiana. These epithelia have in common the fact that they actively transport Cl- at a relatively high rate which accounts for most of their ion transport functions. The various aspects of Cl- transport in these epithelia will be studied with numerous established techniques. These tissues can be mounted as flat sheets in lucite chambers, and the transepithelial electrical and ion transport properties accurately measured. The influences of specific hormones, the effects of certain pharmacological agents, changes with adaptation to different salinities and the influences of other ions on the Cl- transport will be studied with these isolated in vitro preparations. Conventional microelectrode techniques will be employed to obtain potential difference profiles and apical and basal membrane resistance measurements under control conditions as well as after treatment with specific hormonal and pharmacological agents. In addition, chloride-specific microelectrodes will be used to measure the intracellular activities of these ions to determine their distribution across the membranes and to localize the Cl- pump.